Re: [PATCH 2/2] migration/postcopy: map large zero page in postcopy_ram_incoming_setup()

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> On Tue, Oct 08, 2019 at 06:24:23PM +0100, Dr. David Alan Gilbert wrote:
> >* Wei Yang (address@hidden) wrote:
> >> postcopy_ram_incoming_setup() and postcopy_ram_incoming_cleanup() are
> >> counterpart. It is reasonable to map/unmap large zero page in these two
> >> functions respectively.
> >> 
> >> Signed-off-by: Wei Yang <address@hidden>
> >
> >Yes, OK.
> >
> >> ---
> >>  migration/postcopy-ram.c | 34 +++++++++++++++++-----------------
> >>  1 file changed, 17 insertions(+), 17 deletions(-)
> >> 
> >> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> >> index e554f93eec..813cfa5c42 100644
> >> --- a/migration/postcopy-ram.c
> >> +++ b/migration/postcopy-ram.c
> >> @@ -1142,6 +1142,22 @@ int 
> >> postcopy_ram_incoming_setup(MigrationIncomingState *mis)
> >>          return -1;
> >>      }
> >>  
> >> +    /*
> >> +     * Map large zero page when kernel can't use UFFDIO_ZEROPAGE for 
> >> hugepages
> >> +     */
> >> +    mis->postcopy_tmp_zero_page = mmap(NULL, mis->largest_page_size,
> >> +                                       PROT_READ | PROT_WRITE,
> >> +                                       MAP_PRIVATE | MAP_ANONYMOUS,
> >> +                                       -1, 0);
> >> +    if (mis->postcopy_tmp_zero_page == MAP_FAILED) {
> >> +        int e = errno;
> >> +        mis->postcopy_tmp_zero_page = NULL;
> >> +        error_report("%s: Failed to map large zero page %s",
> >> +                     __func__, strerror(e));
> >> +        return -e;
> >
> >Note this starts returning -errno where the rest of this function
> >returns 0 or -1;  returning -errno is a good thing I think and it might
> >be good to change the other returns.
> >
> 
> This is reasonable, while I am thinking how caller would handle this.
> 
> For example, the return value would be finally returned to
> qemu_loadvm_state_main(). If we handle each errno respectively in this
> function, the code would be difficult to read and maintain.
> 
> Would it be good to classify return value to different category? Like
> 
>   POSTCOPY_FATAL
>   POSTCOPY_RETRY
>   POSTCOPY_DISABLE

It's actually quite difficult because unix errno's are too simple;
an EIO has too many causes.
ideally we'd like to be able to separate out a network transport error
that occurs during the postcopy phase for recovery and make sure
that we don't confuse that with any other error.  

Dave

> 
> >
> >Reviewed-by: Dr. David Alan Gilbert <address@hidden>
> >
> >> +    }
> >> +    memset(mis->postcopy_tmp_zero_page, '\0', mis->largest_page_size);
> >> +
> >>      /*
> >>       * Ballooning can mark pages as absent while we're postcopying
> >>       * that would cause false userfaults.
> >> @@ -1248,23 +1264,7 @@ int postcopy_place_page_zero(MigrationIncomingState 
> >> *mis, void *host,
> >>                                             qemu_ram_block_host_offset(rb,
> >>                                                                        
> >> host));
> >>      } else {
> >> -        /* The kernel can't use UFFDIO_ZEROPAGE for hugepages */
> >> -        if (!mis->postcopy_tmp_zero_page) {
> >> -            mis->postcopy_tmp_zero_page = mmap(NULL, 
> >> mis->largest_page_size,
> >> -                                               PROT_READ | PROT_WRITE,
> >> -                                               MAP_PRIVATE | 
> >> MAP_ANONYMOUS,
> >> -                                               -1, 0);
> >> -            if (mis->postcopy_tmp_zero_page == MAP_FAILED) {
> >> -                int e = errno;
> >> -                mis->postcopy_tmp_zero_page = NULL;
> >> -                error_report("%s: %s mapping large zero page",
> >> -                             __func__, strerror(e));
> >> -                return -e;
> >> -            }
> >> -            memset(mis->postcopy_tmp_zero_page, '\0', 
> >> mis->largest_page_size);
> >> -        }
> >> -        return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page,
> >> -                                   rb);
> >> +        return postcopy_place_page(mis, host, 
> >> mis->postcopy_tmp_zero_page, rb);
> >>      }
> >>  }
> >>  
> >> -- 
> >> 2.17.1
> >> 
> >--
> >Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
> -- 
> Wei Yang
> Help you, Help me
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK